AIX-EN-PROVENCE, France—More than 18 months after French President François Hollande unveiled tax incentives to help the country's struggling businesses, Jérôme Frantz is still waiting to reap the benefits.

The French entrepreneur needs the money to help restore the fortunes of his metal-coating company, Frantz Electrolyse. Since demand from car makers collapsed five years ago, revenue has failed to rebound sustainably, margins have...